1. Field Of The Invention
The present invention relates to integrated circuit products which require an external high voltage source to supply programming voltage and current to internal elements. More particularly, the present invention relates to circuitry for multiplexing both high voltage and low voltage sources to a single input pad of an integrated circuit.
2. The Prior Art
Most programmable products require a high external voltage source to supply the voltage and current to program their internal elements. For example a PROM needs an external 20 volt supply to program its memory cells. This programming voltage is normally routed to an internal bus which rests at 5 volts when not programming a cell, and is switched to the higher voltage during programming.
Some of these products have a further limitations in that there are not enough I/O pads to uniquely handle all of the logic inputs, outputs, and power supply inputs, plus this internal programming voltage input. Some of these functions are multiplexed onto one pad in these products. For example, a PROM only needs the 20 volt supply during programming, and does not need chip select during programming; so one pad could handle both functions.
The requirements of switching the power supply connection on an internal power bus, and multiplexing the external power connection with a logic function on a single pad presents some difficult design problems.
Some prior art designs have avoided the multiplexing problem by adding a pad dedicated to supplying only the programming power bus. This approach obviously adds pins to the package which increases its cost to the customer, since it consumes more board space on the product. It may also force multiplexing of other logic signals which could cost speed since the two signals must share the same data path. Therefore, high speed designs with limited pins avoid this approach.
Some prior art products use a design which multiplexes a logic signal and the programming supply onto a single pad. As long as the input signal voltage is below the VDD voltage level of the power supply, power is drawn from V.sub.DD to a node through a diode forward biased between V.sub.DD and the node. When the pad voltage exceeds V.sub.DD, a second diode conducts connecting that internal node to the pad.
While this approach doe function to multiplex the supply and logic signals, it has several practical problems associated with it. First, the voltage on the internal bus depends on the current drawn through the diodes. If no current is drawn, this voltage could drift up to V.sub.DD only to drop immediately when current begins to flow. This internal bus voltage is not regulated and is capable of varying by a diode drop.
Second, noise on the logic signal could alter the internal bus voltage. Since the trip points of the logic mode and power supply mode of the pad are equal to logic high, approximately 5 volts, there is no "dead zone" to provide noise margin.
In addition, the pad can not sink current from the internal power bus, it can only source current because of the diode. So, the internal bus could float higher than either V.sub.DD or the high voltage supply if current were driven into it by for instance a charge pump. A floating power bus could damage devices, cause latch up in CMOS circuits, or cause other circuits to malfunction.
Finally, the diode connected to the pad could significantly increase the pad capacitance, which typically should be less than 5 pF for a logic pin. A programming bus generally must supply a large amount of current. The IR drop of a diode is reduced by increasing its junction area. A large current requires a large junction connected to the pad. The increased capacitance because of the large size of this junction could dominate the total pad capacitance.